1. Field of the Invention
The present invention relates to a positive electrode active material for a secondary battery, a positive electrode for a secondary battery, a secondary battery, and a process for manufacturing a positive electrode active material for a secondary battery.
2. Description of the Related Art
Ten years or more have passed from placing a first lithium ion secondary battery on the market. During the period, mobile devices have been rapidly improved and become widespread. Of course, properties of a lithium ion secondary battery, namely a higher output and a higher energy density, have contributed to such background. As a positive electrode active material for such a lithium ion secondary battery, LiCoO2 has generally been used. LiCoO2 gives a charge-discharge potential of 4 V class in a metal Li counter electrode. LiCoO2 may be relatively easily prepared and give a capacity of about 150 to 160 mAh/g. Thus, it is convenient for constructing a high energy density battery. However, Co as a constituent element of LiCoO2 is expensive. Furthermore, it is not necessarily suitable in terms of long-term reliability for applying it to a large battery for an HEV (hybrid electric vehicle) and the like assuming a longer period of 10 to 20 years.
In addition to requirements for a conventional battery such as good charge-discharge cycle properties at an elevated temperature and good capacity storage properties at an elevated temperature, an HEV application particularly requires prevention of increase of a battery resistance associated with cycles or storage and improved high-rate charge-discharge properties. Because of such situation, a novel positive electrode material in place of LiCoO2 has been needed in a field requiring good high-rate properties and long-term reliability as in an HEV application, and strictly requiring cost reduction.
Recently, an LiNiO2 material having a layered rock-salt structure and an LiMn2O4 having a spinel structure have been investigated in preparation for practical use as a small battery for a mobile device application. An LiNiO2 material has a slightly lower operating voltage, but has a larger charge-discharge capacity of 170 to 200 mAh/g in comparison with LiCoO2. Thus, it may be used to reduce a cost per a unit capacity. However, there are various restrictions for safely using an LiNiO2 material. It has, therefore, not been regarded as the most promising candidate as a next positive electrode active material.
On the other hand, an Li-containing complex oxide represented by LiMn2O4 and having a cubic spinel structure exhibits good high-rate charge-discharge properties because of its crystal structure having three-dimensional Li diffusion paths, is quite safe owing to stability of Mn4+, and is inexpensive. It is, therefore, promising as a positive electrode active material suitable to an HEV application.
However, compared to other layered oxides, LiMn2O4 exhibits larger variation of properties at a high temperature, leading to deterioration in a capacity associated with temperature rise in a charge-discharge cycle or storage.
It is believed that charge-discharge cycle properties in LiMn2O4 are inferior to those in LiCoO2 generally because of Jahn-Teller distortion due to tervalent Mn ions and Mn elution from a lithium manganate crystal into an electrolytic solution. Thus, there has been investigated preparation of an Li-excess composition, that is, Li1+xMn2−xO4, or replacement of an Mn site with another element, particularly Cr (Patent documents 1 and 2).
These techniques basically approximate an Mn valence balance in a lithium manganate to tetravalence to strengthen an oxygen octahedron centering an Mn ion. Improvements in charge-discharge cycle properties using these techniques have been experimentally demonstrated. However, the extent of improvement by these techniques has been insufficient to meet a requirement for a power source for power storage or an electric automobile. Also, it has been insufficient from a viewpoint of a restriction of internal resistance variation shown in a battery premised on an HEV application, therefore, further improvement has been required.
Furthermore, a battery for the HEV application is required to have good high-rate charge-discharge properties, and thus to be of a low resistance and to show small variation in a resistance for a long period. Since investigation in this point of view has not conducted in conventional techniques, the extent of increase in a battery resistance during a long cycle or a long-term storage has not meet a requirement in an HEV or power storage application.
Patent document 1: Japanese Patent Laid-Open No. H6-187993;
Patent document 2: Japanese Patent Laid-Open No. H5-36412.